Known to the art are micromechanical mirror structures (or reflectors) made, at least in part, of semiconductor materials exploiting MEMS (microelectromechanical systems) technology.
MEMS reflectors are designed to receive an optical beam and to vary the direction of propagation thereof, in a periodic or quasi-periodic way. For this purpose, MEMS reflectors include mobile elements formed by mirrors, the positions of which in space are electrically controlled by an appropriate oscillation control signals.
In greater detail, in a generic MEMS reflector comprising a respective mirror, the control of position of the mirror is of particular importance for enabling scanning of a portion of space with an optical beam, which is made to impinge on the mirror. In particular, the control of position of the mirror is crucial in the case of resonant MEMS reflectors, in which, in use, the mirror is made to oscillate in a substantially periodic way about a resting position, the period of oscillation being as close as possible to the resonance frequency of the mirror in order to maximize the angular distance covered by the mirror during each oscillation, and thus maximize the extent of the portion of space scanned.
For example, United States Patent Application Publication No. 2011/0109951 (incorporated by reference) describes a circuit for control of the position of the mirror of a MEMS reflector of a resonant type, said mirror being arranged for rotating, under the action of a motor of an electrostatic type, about an axis of rotation. In particular, the MEMS reflector according to US2011/0109951 comprises a fixed support body, of semiconductor material, and a mirror, which is constrained to the fixed support body by a first spring and a second spring.
The fixed support body comprises a first stator subregion and a second stator subregion, which are connected, respectively, to a first stator electrode and a second stator electrode, and a first rotor subregion and a second rotor subregion, which are connected, respectively, to a first rotor electrode and a second rotor electrode. The first and second stator electrodes enable biasing, respectively, of the first and second stator subregions, whereas the first and second rotor electrodes enable biasing, respectively, of the first and second rotor subregions.
The mirror is mechanically set between the first and second springs, each of which has a respective end that is fixed with respect to the fixed support body. In particular, the first and second springs are fixed, respectively, with respect to the first and second rotor subregions. The mirror and the first and second springs thus form a resonant system, which has a respective mechanical resonance frequency. In general, the mechanical resonance frequency varies in time, for example on account of variations in temperature.
In greater detail, according to the patent application No. US2011/0109951, the voltages of the rotor electrodes and of the stator electrodes, and consequently the voltages of the stator and rotor subregions, are set in such a way as to cause oscillation of the mirror about the axis of rotation. For this purpose, the first and second rotor electrodes are set at a biasing voltage (Vpol), while the first and second stator electrodes receive a same electrical control signal.
To bring about oscillation of the mirror with a mechanical oscillation frequency that is as close as possible to the mechanical resonance frequency, it is necessary to know the mechanical resonance frequency, and it is necessary to generate the pulses of the electrical control signal with an appropriate frequency and phase, as a function of the position of the mirror.
In greater detail, on the basis of the teachings contained in US2011/0109951, it is possible to control the position of the mirror using an electronic driving circuit. In particular, the stator electrodes are considered as being electrically equivalent to, and coinciding with, a node S, and the first and second rotor electrodes are set at the biasing voltage Vpol. The mirror is thus electrically equivalent to a capacitor with variable capacitance Cm, which has a first terminal, electrically coinciding with the node S, and a second terminal, set at the biasing voltage, which is generated by an amplifier according to the virtual-ground principle. The capacitance of the capacitor with variable capacitance Cm depends upon the torsion to which the first and second springs are subjected, and thus is inversely proportional to the angular distance of the mirror from the resting position, where by “resting position” is meant the position of the mirror to which there corresponds a zero torsion of the first and second springs. During driving, the node S receives electrical voltage pulses. Since the electrical pulses are at a high positive voltage having a value higher than the biasing voltage Vpol, a torque of an electrostatic nature is exerted on the mirror; in this way, the mirror is kept in oscillation.
However, application of the biasing voltage Vpol to the first and second rotor electrodes does not make it possible to drive the mirror in oscillation with a pre-set phase. In practice, the oscillation phase is substantially random or, at least in part, subject to offsets that derive from manufacturing steps that may not be determined beforehand.
The knowledge of the oscillation phase is important since the orientation of the mirror defines, in use, the direction of the images projected. Thus, it is necessary to know the orientation (or phase) of the mirror to generate correct images. According to an embodiment of a known type, described in US 2011/0181931, the motion of the mirror is detected by a microphone configured to transduce into an electrical signal the pressure waves generated by oscillation of the mirror. In this way, according US 2011/0181931, it is possible to recognize in which position the mirror is in use, and, consequently, derive the oscillation phase thereof.
Even though this system and method provide reliable results, they require integration of a MEMS microphone in the same package as the one that houses the mirror, with consequent increase in costs and difficulty in containment of the dimension of the final package.
Further, the oscillation phase is not known immediately, but only after oscillation of the mirror has started, at the end of the starting transient.
There is accordingly a need to provide an electrostatically actuated oscillating structure with control of the absolute oscillation starting phase, and corresponding manufacturing method and driving method, that will overcome at least in part the drawbacks of the known art, and in particular will make it possible to govern start of oscillation of the oscillating structure of the MEMS device with a pre-set and known phase.